Beam size measurements based on movable quadrupolar pick-ups

Quadrupolar pick-ups (PU) have been widely studied as candidates for non-intercepting beam size and emittance measurements. However, their application has been proven to be limited. Two fundamental factors make quadrupolar measurements exceptionally challenging: first, the low quadrupolar sensitivity of PUs and second, the parasitic position signal incorporated into the measured quadrupolar measurement. In this paper, an alignment technique, based on movable PUs, is proposed to efficiently cancel the parasitic position signal. Tests have been performed using PUs embedded in collimators in the Large Hadron Collider. Beam measurements demonstrate promising results.peer-reviewe

First operational experience with the LHC Diode ORbit and OScillation (DOROS) System

The LHC started high-energy operation in 2015 with new tertiary collimators, equipped with beam position monitors embedded in their jaws. The required resolution and stability of the beam orbit measurements linked to these BPMs were addressed by the development of a new Diode ORbit and OScillation (DOROS) system. DOROS converts the short BPM electrode pulses into slowly varying signals by compensated diode detectors, whose output signals can be precisely processed and acquired with 24-bit ADCs. This scheme allows a sub-micrometre orbit resolution to be achieved with robust and relatively simple hardware. The DOROS system is also equipped with dedicated channels optimised for processing beam oscillation signals. Data from these channels can be used to perform betatron coupling and beta-beating measurements. The achieved performance of the DOROS system triggered its installation on the beam position monitors located next to the LHC experiments for testing the system as an option of improving the beam orbit measurement in the most important LHC locations. After introducing the DOROS system, its performance is discussed through both, beam and laboratory measurements.peer-reviewe

Final implementation, commissioning, and performance of embedded collimator beam position monitors in the Large Hadron Collider

During Long Shutdown 1, 18 Large Hadron Collider (LHC) collimators were replaced with a new design, in which beam position monitor (BPM) pick-up buttons are embedded in the collimator jaws. The BPMs provide a direct measurement of the beam orbit at the collimators, and therefore can be used to align the collimators more quickly than using the standard technique which relies on feedback from beam losses. Online orbit measurements also allow for reducing operational margins in the collimation hierarchy placed specifically to cater for unknown orbit drifts, therefore decreasing the β and increasing the luminosity reach of the LHC. In this paper, the results from the commissioning of the embedded BPMs in the LHC are presented. The data acquisition and control software architectures are reviewed. A comparison with the standard alignment technique is provided, together with a fill-to-fill analysis of the measured orbit in different machine modes, which will also be used to determine suitable beam interlocks for a tighter collimation hierarchy.peer-reviewe

Upgraded control system for LHC beam-based collimator alignment

In the Large Hadron Collider (LHC), over 100 movable collimators are connected to a three-tier control system which moves them to the required settings throughout the operational cycle from injection to collision energy. A dedicated control system was developed to align the collimators to the beam during machine commissioning periods and hence determine operational settings for the active run. During Long Shutdown 1, the control system was upgraded to allow beam-based alignments to be performed using embedded beam position monitors in 18 newly installed collimators as well as beam loss monitors. This paper presents the new collimation controls architecture for LHC Run II along with several modifications in the Java-based application layer.peer-reviewe

Design and Optimization of the Beam Orbit and Oscillation Measurement System for the Large Hadron Collider

The Large Hadron Collider (LHC) accommodates some 100 collimators whose role is to perform beam cleaning and protect the machine from dangerous particle losses. The collimators are mechanical devices consisting of moveable jaws. Precise positioning and control of the jaws is critical for the cleaning efficiency. Therefore, after the first long shut-down, the LHC was equipped with 18 collimators of a new type. Movable jaws of the new collimators have embedded beam position monitors (BPM) which allow their precise positioning with respect to the circulating beam. However, the existing electronic systems for BPM signal processing could not achieve the required resolution and precision of the position measurements. In addition to the new collimator BPMs, the LHC accommodates more than 1000 BPMs for measuring the transverse positions of the two counter-rotating beams. The standard LHC BPM system uses these BPMs to measure the orbits and oscillations of the beam. The most important BPMs are located next to the LHC experiments. The beam measurements in such locations are the most challenging as the two beams have to be controlled at a fine precision in order to achieve their efficient colliding. Improving the resolution and precision of the position measurements can contribute to the improvement of the machine performance. Performance of the LHC also depends on the magnet optics. Important machine parameters like betatron coupling, beta-beating and phase advance are obtained by exciting the transverse beam oscillations and measuring the amplitudes and phases of the beam response using BPMs. The standard BPM system requires millimetre-order beam excitation to obtain the measurements of a sufficient quality. For machine protection reasons these measurements can be performed only with special beams and dedicated machine set-up. The main task of this doctoral work was to design, prototype, build and optimize a new electronics system for beam position and oscillation measurements in LHC. The system called DOROS (Diode Orbit and Oscillation was primarily designed for the new LHC collimators. The same system was also used to provide high-resolution orbit and oscillation measurements in the selected LHC BPMs. The DOROS systems consist of front-ends, each processing signals from up to two BPM sensors composed of horizontal and vertical transverse plains or to two collimators consisting of upstream and downstream BPMs. The RF signals in a front-end are first filtered, amplified and then split into two diode detector sub-systems which work in parallel. A so called Diode Orbit (DOR) subsystem, based on novel compensated diode detector technique, was designed to perform beam orbit measurements. This thesis describes the analogue processing channels followed by the digital signal processing of the turn-by-turn data and real-time algorithms. The algorithms provide beam based calibration of the channel asymmetries as well as autonomous gain control of the front-end amplifiers. The DOR subsystem was characterized with the laboratory measurements and its performance was demonstrated on a number of beam measurements, showing the achieved sub-micrometre resolution, precision, and long-term stability. The position readings from selected front-ends are also used by the LHC interlock system which terminates operation with beams if the beam positions exceed safe limits. A so called Diode Oscillations (DOS) subsystem, which is based on direct diode detection technique, was designed and optimised to measure small beam oscillations. This thesis describes both the analogue and digital signal processing in a front-end as well as its synchronization and timing circuits. The sampling of the ADCs can be synchronized to the beam allowing to perform precise measurements of the beam coupling and phase advance. The front-end units continuously transmit the measurement readings over Ethernet at 25 Hz rate to the system servers synchronously to the LHC timing. Together with the measurement readings the front- ends transmit also statuses and other data important for diagnostics and reliability of the system. At the same time the acquisition data is stored in parallel to the front-end buffers for detailed turn-by-turn and post-mortem signal analysis

Differential Phase Detector for Precise Phase Alignment

This paper presents a differential phase detector circuit, whose phase-to-voltage characteristic has an extremum when its two input signals are exactly in phase. In this condition all its digital signals are of 50 % duty cycle so that the circuit characteristic does not have a dead zone. This feature allows a precise indication of the zero-phase condition, which is independent of the detector power supply and the offset of its ADC readout. Such a detector is used for a phase alignment of two reference clock signals with frequency about 11 kHz in front-ends processing signals from beam position monitors of the Large Hadron Collider (LHC) at CERN. The detector output voltage is digitized with a 24-bit ADC at the rate of the reference signals. The resulting samples are processed in the front-end FPGA and transmitted to the control system using an Ethernet data stream. After a detailed description of the differential phase detector its performance is demonstrated with laboratory measurements. The results show that this simple circuit allows a phase alignment resolution of the 11.2 kHz clock signals in the order of 0.0001 º with a measurement bandwidth of 1 Hz

Differential phase detector for precise phase alignment

This paper presents a differential phase detector circuit, whose phase-to-voltage characteristic has an extremum when its two input signals are exactly in phase. In this condition all its digital signals are of 50% duty cycle so that the circuit characteristic does not have a dead zone. This feature allows a precise indication of the zero-phase condition, which is independent of the detector power supply and the offset of its ADC readout. Such a detector is used for a phase alignment of two reference clock signals with frequency about 11 kHz in front-ends processing signals from beam position monitors of the Large Hadron Collider (LHC) at CERN. The detector output voltage is digitized with a 24-bit ADC at the rate of the reference signals. The resulting samples are processed in the front-end FPGA and transmitted to the control system using an Ethernet data stream. After a detailed description of the differential phase detector its performance is demonstrated with laboratory measurements. The results show that this simple circuit allows a phase alignment resolution of the 11.2 kHz clock signals in the order of 0.0001° with a measurement bandwidth of 1 Hz

First Operational Experience with the LHC Diode ORbit and OScillation (DOROS) System

The LHC started high-energy operation in 2015 with new tertiary collimators, equipped with beam position monitors embedded in their jaws. The required resolution and stability of the beam orbit measurements linked to these BPMs were addressed by the development of a new Diode ORbit and OScillation (DOROS) system. DOROS converts the short BPM electrode pulses into slowly varying signals by compensated diode detectors, whose output signals can be precisely processed and acquired with 24-bit ADCs. This scheme allows a sub-micrometre orbit resolution to be achieved with robust and relatively simple hardware. The DOROS system is also equipped with dedicated channels optimised for processing beam oscillation signals. Data from these channels can be used to perform betatron coupling and beta-beating measurements. The achieved performance of the DOROS system triggered its installation on the beam position monitors located next to the LHC experiments for testing the system as an option of improving the beam orbit measurement in the most important LHC locations. After introducing the DOROS system, its performance is discussed through both, beam and laboratory measurements

Summary of LHC MD 369: DOROS vs WBTN in IR Stripline BPMs

The aim of this MD is to quantify the impact of the stripline beam position monitor (BPM) directivity with two acquisition chain electronics systems, WBTN (Wide Band Time Normalizer) and DOROS (Diode ORbit and Oscillation System). This impact depends on the relative position and intensity of the two beams at the location of the monitor. This note explains all the procedures of the LHC MD 369, which took place on 20/07/2015 and presents the obtained results

Upgraded Control System for LHC Beam-Based Collimator Alignment

In the Large Hadron Collider (LHC), over 100 movable collimators are connected to a three-tier control system which moves them to the required settings throughout the operational cycle from injection to collision energy. A dedicated control system was developed to align the collimators to the beam during machine commissioning periods and hence determine operational settings for the active run. During Long Shutdown 1, the control system was upgraded to allow beam-based alignments to be performed using embedded beam position monitors in 18 newly installed collimators as well as beam loss monitors. This paper presents the new collimation controls architecture for LHC Run II along with several modifications in the Java-based application layer